Touch sensitive overlays (TSO) are encountered frequently in use with keyboards or touch responsive computer monitor screens. They typically involve at least two layers of suitable flexible plastic material maintained very closely spaced and parallel to each other, with each one of the adjacent surfaces being coated in a suitable pattern with electrically conducting material such as, for example, vacuum deposited gold. When external pressure, as by the finger of an operator, is applied to one of the sheets the gap between the two is closed and electrical contact is made between the patterned electrically conducting materials thus providing a useful signal which can be decoded. There is, generally, a clear firm surface provided behind the flexible electrically conducting coated layers to provide support when these are pressed together. A convenient material for this use is acrylic plastic. The different layers are all closely spaced apart and held firmly on the outer peripheries. The electrically conductive portions are electrically connected to a suitable circuit to form an operating TSO.
In many uses, e.g., when light from a computer monitor is observed through a conventional TSO, it is noticed that Newton rings are formed around the point at which the person presses the TSO to obtain the signals. Depending on the transparent materials, the layer thicknesses, and the spacing between adjacent surfaces, Newton rings may be seen even without the application of a local force. Newton rings are multicolored striations that are visible, e.g., around the point at which the TSO is pressed, and are caused by interference between light waves passing through the very narrow thin regions around a point of close proximity between transparent materials. When Newton rings are formed in a location so as to intrude on or intercept a visual or optically perceptible signal coming through the TSO they distract the operator and are deleterious to the operation of the TSO for prolonged periods. It is therefore desirable that a simple, convenient, and inexpensive solution be found to reduce or eliminate the formation of Newton rings within TSO assemblies.
It is old in the art to deliberately apply non-conducting material in a selected pattern, or to have small raised points or bumps in one or both surfaces that have electrically conducting layers applied to them as part of the TSO assembly. Such layers of panel 11, and the various electrical connections 12, 13 to different electrically conducting layers, are conventionally held together by metal clips 60 applied at the peripheral edges of the panel (see FIG. 1). What is contemplated here, however, is the avoidance of the formation of Newton rings behind the rearmost surface of such a layer, between it and a transparent backing plate forming part of the TSO assembly. The present invention teaches a solution that involves a deposition of fine particulate material, with the particles being sized within a predetermined range, to thereby significantly reduce or avoid the formation of undesirable Newton rings as described above.